Despite summertime temperatures that turned nest boxes into ovens, there were more eggs laid at the Cache Creek Nature Preserve this past year, even though a number of chicks didn't survive.
According to a study done by Cache Creek Conservancy biologist Felicia Wang, 2023 "was a successful bird breeding season, with the number of eggs, nestlings, and fledglings being comparable to 2022."
On the face of it, that doesn't appear to be very good. However, the news comes despite high mortality rates caused by extreme temperatures in the Preserve that started in June and continued through August.
It also signifies a healthier ecosystem.
Wang reported that the first nesting attempts occurred from March 15 to May 24. During this period, 97 eggs were laid, 83 eggs hatched, and 78 nestlings fledged, or an 85.6% hatch rate and a 94% fledgling success rate.
"Re-nest attempts occurred from May 24 to August 3," Wang continued. "Not all nest boxes experienced a second brood, but most did (13 out of 15)."
During these three months, 62 eggs were laid, 53 eggs hatched, and 23 nestlings fledged. This produced an 85.5% hatching rate and a 43% fledgling success rate. The re-nest fledgling success rate was markedly lower than the first nesting attempt. The primary reason for this is thought to be the difference in air temperature.
Wang reported the average high temperature during the first nesting was around 71 degrees. While the second nesting temperature averaged 93.5 degrees, with many days later in the season reaching 100 degrees or higher.
"Compared to natural tree cavities, nest boxes are less insulated and are therefore more sensitive to outside ambient temperatures," Wang noted. "Internal box temperatures can reach up to 45 degrees warmer than tree hollows. The egg hatching rate may not have been impacted because most eggs were laid earlier before daytime temperatures were consistently warm.
"Late June is when summer temperatures began to increase, which was also the time when most nest boxes were occupied by week-old nestlings. Nestlings are particularly threatened by heat due to their immobility, rapid growth, and immature physiology. Considering all of the dead nestlings were found inside boxes with less shade cover, and therefore more sun exposure, heat stress is thought to be the leading factor in nestling death and the low fledging success rate."
Yet, despite the increased deaths, there were more eggs laid and hatched in 2023 than 2022, even though there was one less nest box. In total, there were 159 eggs laid and 136 eggs hatched, but only 101 of those nestlings fledged, or a rate of 74%.
In 2022, when there were 145 eggs laid, 130 nestlings, and 111 fledglings, resulting in a fledgling success rate of 85.40%.
The fledgling success rate in 2023 can probably be accounted for because many of the eggs were laid during the first part of the year as well as a higher number of breeding birds, which Wang said indicates a "healthy riparian habitat."
Wang noted that even in the face of success, there are always additional habitat recommendations and points of improvement that can be made.
"Perhaps the issue that is most prominent is heat stress within the nest boxes," she noted. "With climate change and the anticipation of warmer weather in the future, it is important to address these threats to ensure that the nest boxes remain safe nesting environments. Box modifications, such as painting the exterior a lighter shade or building an overhead shade structure, can help mitigate these concerns. Nest boxes can also be re-located so that they are placed under existing shade cover."
"Mature trees act as food sources and shelter from heat and predators. Perhaps the addition of more mature trees would increase nest box occupancy rates so that all boxes are utilized, though the current rate of 86.70% is still good," she added.
In terms of which birds benefited the most, Wang reported that tree swallows continue to make up the majority of nest box users at the Nature Preserve. Not only did tree swallow box utilization remain strong, it increased in 2023. Species diversity and evenness decreased at the Preserve. Some 96% of fledglings were tree swallows in 2023, compared to 89% in 2022.
"It is likely that the increased presence of water, through a heavier than usual rain year, made the Nature Preserve even more desirable for tree swallows. There was abundant water in the wetlands, Cache Creek, and adjacent canals and sloughs. Tree Swallows are insectivorous birds and rely on catching their food, for themselves and their young, above the surface of the water.
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